TOUGHENING OF EPOXY-RESIN USING HYDROXYL-TERMINATED POLYESTERS

Epoxy resins are increasingly finding applications in the field of str uctural engineering. A wide variety of epoxy resins are available, and some of them are characterized by relatively low toughness. Several a pproaches to improve epoxy resin toughness include the addition of fil lers, rubber particles, thermoplastics, or their hybrids, as well as i nterpenetrating networks and flexibilizers, such as polyols. It seems that this last approach did not receive much attention. So in an attem pt to fill this gap, the present work deals with the use of hydroxyl-t erminated polyester resins as toughening agents for epoxy resin. For t his purpose, the modifier, that is, a hydroxyl terminated polyester re sin (commercially referred to as Desmophen), which is a polyol, has be en used at different concentrations. The prepared modified structure h as been characterized using Fourier transform infrared (FTIR) spectros copy and scanning electron microscopy (SEM) prior to mechanical testin g in terms of impact strength and toughness. Two types of Desmophen (8 00 and 1200) have been used as modifiers. The obtained results showed that hydroxyl-terminated polyester improves the epoxy toughness. In fa ct, the impact strength increases with Desmophen content and reaches a maximum value of 7.65 J/m at 10 phr for Desmophen 800 and 9.36 J/m at 7.5 phr for Desmophen 1200, respectively. At a critical concentration (7.5 phr), Desmophen 1200 (with higher molecular weight, longer chain s, and lower branching) leads to better results. Concerning K-c, the e ffect of Desmophen 800 is almost negligible; whereas a drastic effect is observed with Desmophen 1200 as K-c reaches a maximum of 2.41 MPa m (1/2), compared to 0.9 MPa m(1/2) of the unmodified epoxy prior to dec reasing. This is attributed to the intensive hydrogen bonding between epoxy and Desmophen 1200, as revealed by FTIR spectroscopy. Finally, t he SEM analysis results suggested that the possible toughening mechani sm for the epoxy resin being considered, which might prevail, is throu gh localized plastic shear yielding induced by the presence of the Des mophen particles. (C) 1999 John Wiley & Sons, Inc.